Learning ObjectivesBy the end of the lesson you should be able to:

Label a sperm and ovumDescribe a zygote Explain what happens after fertilization

Gametes – e.g. sperm or egg (ovum)

Gametes is the name given to sex cellsMen produce spermWomen produce eggs, OvaSperm are produced in great numbers in the testes (millions every day)Ova mature one at a time in the ovaries (takes 28 days)

Sperm cell

The sperm cell has 23 unpaired chromosomes from the father, these are contained in the nucleus (3)The head has an enzyme filled acrosome (1), this breaks through the zona pellucida of the ovumThe mid-piece (4) contains mitochondria to provide energy for the tail.The tail (flagellum) (5) thrashes to propel the sperm forward

Ovum

This large structure has plenty of nutrient rich yolk in its cytoplasmIt’s nucleus is bigger but still only contains 23 chromosomes from the motherThe zona pellucida is a jelly layer through which the sperm must break to reach the nucleus of the eggThere may also be old follicle cells stuck to the egg

The zygote

Once a sperm has fused with an egg it is known as a zygote In a human the 23 unpaired chromosomes from the sperm join with the 23 unpaired chromosomes in the ovum This makes the full human complement of 46 chromosomes forming a unique zygote.

Fertilization

Once the zygote is formed fertilization is complete, it is still not guaranteed that pregnancy will take place.If a zygote is formed the corpus luteum will not degenerate (why is this useful?)The zygote finds its way into the uterus and implants (pregnancy has begun)

First cell divisions

As the zygote travels along it begins to divide.The yolk makes this possibleThe one becomes two, the two become four etc.By the time it reaches the uterus the yolk is used up and the zygote must implantIf the dividing zygote completely splits at this early stage identical twins are formed.

First 3 months

The Progesterone and Oestrogen inhibit Follicle Stimulating Hormone (FSH) production (why?)Progesterone and oestrogen also cause the breasts to enlargeThe corpus luteum finally degenerates after 3 monthsBy this time the placenta is fully formed (it takes over the role of the corpus luteum in producing progesterone and oestrogen)